It is well known to use a fluorescent brightening agent in order to heighten whiteness of the background of silver halide photographic materials after processing. A fluorescent brightening agent absorbs ultraviolet rays and generally fluoresces a light blue color, to thereby impart apparent whiteness to an object.
Such a whitening method conventionally includes a method of adding a fluorescent brightening agent to a paper support or a polyolefin layer of the paper support, a method of adding a water-soluble or oil-soluble fluorescent brightening agent to a silver halide emulsion layer or other photographic coating layers, and a method of previously adding a fluorescent brightening agent to a developing solution. However, incorporation of the fluorescent brightening agent into a laminate layer of a polyethylene laminate paper is disadvantageous in that the fluorescent brightening agent is apt to decompose due to its poor heat stability during heat-extrusion for lamination. Further, attempts to impart fluorescent whiteness during development procesing by the addition of the fluorescent brightening agent to a developing solution are disadvantageous since uniformity in quality of the final product cannot be maintained unless the concentration of such a fluorescent brightening agent is maintained constant throughout the developing processing.
When considering incorporation of a fluorescent brightening agent in an image-receiving layer of an image-receiving material, it is more effective to use an oil-soluble fluorescent brightening agent rather than a water-soluble fluorescent brightening agent, from the viewpoint of preventing the agent from running out during processing or washing with water. To achieve this effect, a method has been proposed in which an oil-soluble fluorescent brightening agent is dissolved in an organic solvent to obtain an emulsified dispersion and the dispersion is added to the gelatin layer, as disclosed, e.g., in British Pat. No. 1,072,915. However, this proposal still has disadvantages, such as that incorporation of such an emulsified dispersion in an image-receiving layer results in reduction of transferred density, or fluorescence is quenched to lose the whitening effect when the image-receiving material is treated with a processing solution containing an amine compound.
On the other hand, in the case where a water-soluble fluorescent brightening agent is incorporated in an image-receiving layer, particularly when the proportion of the water-soluble fluorescent brightening agent to gelatin in the image-receiving layer is great, it was found that not only does the fluorescent brightening agent easily escape from the image-receiving layer during processing and washing with water, but also introduction of the agent reduces the transferred density.
Further, even if a fluorescent brightening agent is introduced in an image-receiving layer, treatment with a processing composition containing an amine compound quenches the fluorescence to destroy the whitening effect.
It is an important requirement for a fluorescent brightening agent not to escape during washing with water, taking into account that an image-receiving material, after processing, is washed with water for a period as long as 5 to 10 minutes in many cases.
Accordingly, it has been strongly desired to develop a method of whitening an image-receiving material which does not lose its whitening effect during development processing or washing with water and also is not accompanied by a reduction of transferred density. Moreover, a method for improving the whiteness of an image-receiving material has been keenly desired in a diffusion transfer process wherein a processing solution containing an amine compound is used.